An optical information recording medium including a thin film formed on a transparent substrate, which enables recording/reproducing of an information signal by irradiating this thin film with a laser beam narrowed into a minute spot, is well-known. As a write-once type optical information recording medium, one including a TeOx(0<x<2) thin recording film formed on a substrate is known (See JP S50-46317A, for example), where TeOx is a mixture of Te and TeO2. This optical information recording medium allows a larger degree of reflectivity change to be attained by the irradiation with a laser beam for reproduction.
With the use of the TeOx thin recording film, initialization such as laser annealing can be eliminated, and a crystalline recording mark can be formed by irradiating the film that is in an amorphous state after the film formation with a laser beam. This is an irreversible procedure, so that correction and erasure by overwriting cannot be conducted. Therefore, a medium using this thin recording film can be used as an optical information recording medium enabling write-once only.
In the TeOx thin recording film, a certain duration of time is required until a signal becomes saturated after recording, i.e., crystallization in the thin recording film by the irradiation with a laser beam proceeds sufficiently. Therefore, the optical information recording medium using the TeOx thin recording film without any treatment is inappropriate for a recording medium required to have a quick response property, such as a data file for a computer in which data is recorded on a disk and the data is to be verified after one rotation of the disk. In order to compensate for this drawback, it has been suggested that Pd, Au or the like is added as the third element to the TeOx thin recording film (See JP S60-203490A, JP S61-68296A and JP S62-88152, for example).
Pd and Au are considered to have a function of promoting the crystal growth of Te in the TeOx thin recording film during the irradiation with a light beam. The addition of Pd or Au enables the generation of crystal grains of Te and Te—Pd alloy or Te—Au alloy at a high speed. Furthermore, since Pd and Au have good resistance to oxidation, these elements will not degrade the good moisture resistance of the TeOx thin recording film.
Meanwhile, as a basic means for increasing the amount of information that one optical information recording medium can deal with, there is a method for shortening the wavelength of a laser beam or increasing the numerical aperture of an objective lens for collecting the laser beam so as to decrease a spot size of the laser beam, thus enhancing a recording surface density. Furthermore, in order to enhance the recording density in the circumferential direction, mark edge recording has been proposed and introduced in which a length of a recording mark represents information. Moreover, in order to enhance the recording density in the radius direction, land & groove recording has been proposed and introduced, in which recording is performed with respect to both of a groove for guiding a laser beam and a land between the grooves. As a further means for increasing the amount of information, a multilayer structured optical information recording medium including a plurality of laminated information layers and a method for recording and reproducing with respect to the same also have been proposed (See JP H9-212917A, JP H10-505188A and JP 2000-36130A, for example).
In order to be ready for such high-density recording, an optical information recording medium with an improved composition of a recording material of a TeOx thin recording film containing Pd, Au or the like added thereto as a third element and an improved film thickness has been proposed for a write-once type optical information recording medium (See WO98/09823A1, for example).
An important challenge for making multilayer structured optical information recording media practical is to enhance the recording sensitivity. Optical information recording media generally adopt a general-purpose laser diode as a light source for recording/reproducing, so that the recording should be performed within a limited laser power output.
In the multilayer structure, however, recording/reproducing are performed by using a laser beam that is incident from one side to the plurality of information layers, and therefore recording is performed on an information layer located farther from a laser beam incident side with a laser beam that is attenuated during the passage through information layers located closer to the laser beam incident side. Therefore, an information layer disposed farther from the laser beam incident side is required to have a higher recording sensitivity. On the other hand, an information layer located closer to the laser beam incident side is required to have a higher transmittance.